Fetal hemoglobin (HbF) inhibits the polymerization of HbS and is therefore an important modulator of the phenotype of sickle cell anemia. In this disease, HbF concentrations span two orders of magnitude. Hydroxyurea can prevent some of the vasoocclusive complications of sickle cell anemia, an effect that is due, at least partially, to it s ability to increase HbF levels. But, patients who take hydroxyurea to increase HbF levels have an unpredictable response. We hypothesize that these observations are explained by genetic variation in: cis- and trans-acting elements implicated in gamma-globin gene expression; enzymes controlling hydroxyurea pharmacokinetics and pharmacodynamics; erythroid progenitor proliferation; unknown genes that directly or indirectly modulate HbF concentration. To define this genetic heterogeneity, we will use high throughput screening by mass spectrometry to: discover single nucleotide polymorphisms (SNPs) and haplotypes that predict the HbF level in sickle cell anemia patients taking hydroxyurea and discover SNPs and haplotypes associated with 'baseline HbF concentrations. We also hope to discover new genes that regulate HbF level and the HbF response to hydroxyurea. Nearly 300 sickle cell anemia patients treated with hydroxyurea and an additional 150 untreated sickle cell anemia patients who have steady-state HbF levels measured, make up our unique patient base. Our approach to SNP detection has 3-phases. In Phase 1, we examine SNPs in about 50 candidate genetic loci that plausibly modulate the HbF response to hydroxyurea and baseline HbF concentration. These loci include; genes regulating hydroxyurea metabolism, governing cellular hydroxyurea action, regulating hematopoiesis, transcription factors suspected to modulate globin gene expression and putative regulatory elements linked to the beta-globin gene cluster. In Phase 2, our study is expanded to examine more than 200 biologically important genes, including; transcription factors and regulators like DNA dependent ATPases, PHD finger domains, ATRX-like genes, apoptosis genes, regulators of cell proliferation, methyl tranferases; histone acetylase/deacetylases, HOX genes and loci implicated in HbF regulation. In Phase 3, we pick genes and SNPs based on their location on the genome map, their informativeness and their possible biological importance. The 3 Phases of our study are complementary. HbF regulation is a polygenic trait and our overarching aim is to define as completely as possible genetic modifiers of HbF expression. Identification of promising genes and SNPs in phases I and 2 will suggest other genes for further studies. About 2000 genes will be studied in Phases 2 and 3 approximating a whole genome scan at 1.5 cM resolution. We hope ultimately to discover SNPs, haplotypes or genes that predict or modulate the HbF response to hydroxyurea and better define HbF regulation in sickle cell anemia.